Both measure how hard/easy to make a fluid flow. What additional information does Deborah’s number provide that makes counting on viscosity alone insufficient?
In: 0
Viscosity is a property of a material like density, thermal conductivity and so on.
Deborah’s number like most dimensionless numbers is a ratio of two things. It is dictated by both the viscous and elastic behaviour of a material. Viscoelastic materials can behave like a solid or a fluid depending on how deformation is applied to it. Viscosity doesn’t capture that elastic part, Deborah’s number does.
De is the ratio of a characteristic time if whatever process you’re characterizing and the material’s relaxation time. The process characteristic time is a measure of what’s happening to the material. The relaxation time is a property of the material. Just with that, it is quite different from viscosity as it captures what’s happening to the material and properties of the material.
Say for example, you wanted to do something with polydimethylsiloxane (PDMS). If you apply deformation to it very quickly, it behaves like a solid. If you go at it slowly, it behaves more like a viscous liquid. Calculating Deborah’s number for that process will let you know at a glance how it will behave.
If you wanted to use viscosity for that, you’d have to figure out what the strain rate on the material would be and know how the viscosity of the material behaves as a function of strain rate.
Latest Answers